Smooth Water Surface Areas?

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I look out at a channel between two large islands in a sea. On near windless days much of the sea is smooth, but then there are trails that are even smoother. Unconsciously I thought these were boat wakes, long after the boat has passed. But I see some of them are definitely not that. Some are perhaps 10 meters across and they last for long periods of time. Some also are regions and lack the trail form. What are these? or perhaps better, what might these be?